1. Field of the Invention
The present invention generally relates to wastewater treatment apparatus and processes, and more particularly to an apparatus and method for filtering suspended solids from liquid effluent during a tertiary filtration phase in wastewater and sewage treatment applications.
2. Description of Related Art
Sewage and wastewater treatment generally refers to the process of removing contaminants from wastewater. Sewage is created by residences, institutions, hospitals, and industrial facilities, and includes household waste disposed of via sewers. Sewage is typically collected and transported via a network of pipes to sewage treatment plants wherein physical, chemical, and biological processes are employed to remove contaminants.
Modern sewage treatment involves three stages, namely, primary, secondary, and tertiary treatment. Primary treatment generally involves temporary retention of the sewage in a basin to allow heavy solids to settle to the bottom while oil, grease, and lighter solids float to the surface. In the primary sedimentation stage, sewage flows through large tanks, commonly called “primary clarifiers” or “primary sedimentation tanks.” Settled and floating substances are removed and the remaining liquid is subjected to secondary treatment. The secondary treatment stage degrades the biological content of the sewage. The majority of municipal sewage treatment plants treat the settled sewage liquor using aerobic biological processes, such as the use of water-borne micro-organisms to remove dissolved and suspended biological matter. Biological treated waters flow to a secondary sedimentation stage for clarification. Tertiary treatment involves final treatment stages that raise the effluent quality prior to discharge to the receiving environment, typically achieving water quality of 25-30 parts per million of total suspended solids (TSS). More than one tertiary treatment process may be used at any treatment plant.
Gravity settler tanks and clarifiers have long been used in sewage and wastewater treatment to separate solids from liquids in effluent streams in tertiary treatment applications. One particular type of settler tank known in the art is characterized as having a series of inclined solid plates that function to increase the settlement velocity thereby enhancing the efficiency at which flocculated material precipitates from water. One such brand of inclined plate technology is branded under the trademark LAMELLA®, a registered trademark of Parkson Corporation, Fort Lauderdale, Fla. So called, LAMELLA clarifiers receive influent that flows upward over an array of inclined parallel solid plates whereby solids tend to settle on the plates and eventually slide into a sludge hopper at the bottom of the settler tank. It has been found that use of LAMELLA clarifiers require a smaller surface footprint as compared with conventional clarification equipment to achieve a given solids removal capacity because the inclined plates enhance solids settlability.
Another wastewater treatment technology present in the wastewater treatment art involves the use of porous media filters, such as woven fabric, knitted fabric, gauze, mesh, penetrable membranes etc. These systems are often referred to as disk or drum filtration because they are characterized as having a tank with an inlet and an outlet and filter media disposed between the inlet and the outlet, wherein the filter media typically comprises a cloth media stretched over large drum or disk-type frames. In these systems, liquid influent is filtered upon passing, under hydrostatic pressure, through the porous woven cloth filter media which captures solid particles as the liquid passes through the filter media. Over time filtered particles accumulate on the filter media thereby degrading the effectiveness of the filter. As the filtration process continues, solids accumulating on the filter media will gradually restrict the flow of liquid through the media causing the level within the filter tank to rise. When the level reaches a predetermined level, a backwash process is required to restore filter effectiveness by cleaning the filter media.
The background art reveals a number of attempts to adapt tertiary media filters with systems intended to periodically clean the filter media by removal of accumulated solids. This process is sometimes referred to as “backwashing”. There are two primary competing designs for backwashing systems known in the art for use with tertiary media filtration systems. Each tertiary filtration system is generally characterized as having a plurality of filters, typically disk-shaped, submerged in effluent within a tank. The tertiary filtration systems typically have disk-shaped filters disposed generally vertically and supported by a horizontally disposed axel. Each of these competing prior art designs uses a vacuum pump to create suction for vacuum heads disposed externally on each side of each filter disks to remove captured solids from the media. One design is characterized by having filter disks that to rotate while fixed (radially elongate) vacuum heads disposed on opposite sides of the filter remove accumulated solids from the filter media. The other design is characterized by having dynamic vacuum heads that are caused to move across the surfaces of fixed filter disks. An example of such technology is found in Patent Application Publication No. US 2005/0139557, titled Tertiary Filter.
Both the fixed disk/movable vacuum head, and the fixed vacuum head/rotatable disk backwashing technologies, as well as other media filters that incorporate suction backwashing, are burdened with significant disadvantages. Initially, turbidity spikes are experienced during suction backwashing processes. One reason for the turbidity spikes is believed to result from the cleaning of the external surface of the filter which removes significant amounts of deposited particulate matter (i.e. accumulated solids) thereby reducing the effectiveness of the filter for a period time until the accumulation of additional particulate matter raises filtration efficiency to an acceptable level. During the period of reduced effectiveness, flow increases through the recently cleaned portions of the filter thereby allowing an increased amount of solids to pass through. The present inventor has found that it is desirable to leave a suitable amount of accumulated deposited solids on the filter media during the backwashing process so as to maintain filtration efficiency. In addition, the application of an external suction force on the outer surface of the filter media reduces the internal pressure within the filter disk thereby artificially increasing the flow of water through the remaining portions of the filter. The reduced internal pressure and resulting sudden increase in flow results in increased levels of solids passing through the filter during the backwashing process thereby leading to a spike in turbidity corresponding to each backwashing cycle. More particularly, the high suction causes the counter-current condition and allows solids to pass into the clean water volume causing the “cleaning spikes”. A further disadvantage with tertiary media filter systems present in the art is that the filter media is configured in generally vertically disposed arrays and thus fail to harness advantages realized by inclined plate technology. Furthermore, the vacuum cleaning heads take up a great deal of space thereby limiting the number of discs per tank. In addition, such systems require substantial energy consumption to power the vacuum cleaning system.
Accordingly, there exists a need for an improved tertiary media filtration system with backwash capabilities that avoids the limitations and disadvantages present in the art.